Fluororesin, method of reforming fluororesin, sliding member, and sliding device

ABSTRACT

There is provided a fluororesin which has a low friction coefficient, superior wear resistance and a superior friction characteristic in the presence of a lubricating oil and/or an operating oil. The fluororesin has a structure in which some of the hydrogen atoms and/or fluorine atoms bonded to carbon atoms of a main frame resin are substituted with an oxygen-containing functional group. Visible light transmittance in a wavelength of 600 nm is increased as compared with that of the main frame resin. There is also provided a method of reforming a fluororesin, comprising extracting some of the hydrogen atoms and/or fluorine atoms bonded to carbon atoms of a main frame resin to introduce an unsaturated bond; and applying dissociation energy of the unsaturated bond in the presence of oxygen. There are further provided a sliding member, a sliding apparatus and a sliding system.

This application claims the benefit of Japanese Application No.2004-187190, filed Jun. 25, 2004, the entirety of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to a fluororesin which has both lowfriction coefficient and excellent wear resistance, particularly to afluororesin preferably for use under lubrication by a lubricating oil oran operating oil, and a sliding member and a sliding device comprisingsuch a fluororesin.

BACKGROUND OF THE INVENTION

Since fluororesins have low friction coefficients, they heretofore havebeen used in many sliding portions. However, since fluororesins have aband structure, they are generally inferior in wear resistance. As ameasure for enhancing the wear resistance of the fluororesin, it iscommon to add fiber-based fillers such as glass fibers and carbon fibersor metal fillers such as bronze. However, when these fillers are added,the fillers promote wear on a sliding target material in cases where thematerial is a nonferrous metal such as an aluminum alloy. On the otherhand, when the addition of the filler to the fluororesin is limited inorder to reduce the wear on the target material, predetermined wearresistance is not obtained.

As a means for solving the above-described problem, there has beenprovided a reformed fluororesin which has been reformed by irradiatingthe fluororesin with electrolytic dissociative radiation while beingheated under an inactive gas atmosphere at a temperature equal to itsmelting point or higher (see, Japanese Patent Application Laid-Open NO.11-116624). The wear resistance of this reformed fluororesin can belargely enhanced without adding the fillers thereto. Therefore, asuperior sliding characteristic is indicated under dry lubrication.

Additionally, since the reformed fluororesin exhibits low surfaceenergy, and exhibits an oil-repellent property in the same manner as ausual fluororesin, generation of an oil film is inhibited in thepresence of the lubricating oil or the operating oil, and an effect ofsufficiently reducing the friction coefficient is not obtained.

Although the wear resistance of the fluororesin is improved in thismanner, an oil film is inhibited from being formed in the presence ofthe lubricating oil and the operating oil, and a fluid lubricatingeffect by the lubricating oil and the operating oil cannot be easilydeveloped, because the fluororesin has the low surface energy andexhibits the oil-repellent property.

To solve this problem, as methods of improving wettability with thelubricating oil and the operating oil, a corona treatment, an RFbombardment treatment, a DC bombardment treatment (see InternationalPublication No. WO 98/44026), a method of introducing monomer other thanmonomer constituting the fluororesin as a functional group (see JapanesePatent Application Laid-Open 2001-208249), and a plasma treatment havebeen known.

SUMMARY OF THE INVENTION

However, since a special step for increasing surface energy is requiredin any of the above-described methods, there has been a problem thatcosts rise.

Moreover, when a method of reforming a fluororesin by theabove-described electrolytic dissociative radiation treatment, and amethod of improving wettability with a lubricating oil and operating oilare used together, it is theoretically possible to manufacture afluororesin exhibiting a superior friction characteristic in thepresence of the lubricating oil and operating oil. However, two types ofmanufacturing steps have to be added, a cost aspect raises a problem,and therefore this method has not been put into practical use in thepresent situations.

The present invention has been developed in view of the problems of theconventional techniques, and objects thereof are to provide afluororesin which has both a low friction coefficient and a superiorwear resistance and which exerts a superior friction characteristic inthe presence of a lubricating oil and/or an operating oil, a method ofreforming a fluororesin, a sliding member comprising a sliding portioncomprising such a fluororesin, a sliding apparatus comprising such asliding member and at least one oil material, and a sliding systemcomprising such a sliding apparatus and a target material comprising asliding surface on which the sliding member slides.

As a result of intensive studies performed in order to address theabove-described objects, the present inventors have found that theabove-described objects can be achieved, when a predeterminedoxygen-containing functional group is introduced by an appropriateunsaturated bond introduction process or the like, and they havecompleted the present invention.

That is, according to the present invention, there is provided afluororesin comprising a substituted main frame resin on which at leastone of the hydrogen atoms and/or fluorine atoms bonded to carbon atomsof a main frame resin are substituted with an oxygen-containingfunctional group and in which the visible light transmittance in awavelength of 600 nm of the fluororesin is greater than that of the mainframe resin.

Moreover, there is provided a method of reforming a fluororesin,comprising: extracting some of the hydrogen atoms and/or fluorine atomsbonded to carbon atoms of a main frame resin to introduce an unsaturatedbond to produce a modified main frame resin; and applying dissociationenergy of the unsaturated bond in the presence of oxygen.

Furthermore, according to the present invention, there is provided asliding member comprising a fluororesin as described above in a slidingportion. Additionally, according to the present invention, there isprovided a sliding apparatus comprising a sliding member as discussedabove and at least one oil material selected from among lubricating oiland operating oil. In addition, according to the present invention,there is provided a sliding system comprising a sliding apparatus asdiscussed above and a target material which comprises a sliding surfaceon which the sliding member of the sliding apparatus slides.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description of apreferred mode of practicing the invention, read in connection with theaccompanying drawings in which:

FIG. 1 is a graph showing a change of a visible light transmittance in acase where a process of introducing an unsaturated bond and a process ofapplying dissociation energy of the unsaturated bond according to thepresent invention are performed with respect to a tetrafluoroethyleneresin.

FIG. 2 is a perspective view showing a shape of a ring test piece usedin a friction test.

FIG. 3 is a schematic diagram of a vertical ring on a disc typefrictional wear tester used in a friction test.

FIG. 4 is a graph showing a change of a friction coefficient with elapseof time.

FIG. 5 is a schematic drawing of an embodiment of a sliding system inaccordance with the present invention.

FIG. 6 is a schematic drawing of another embodiment of a sliding systemin accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A fluororesin of the present invention will be described hereinafter indetail. It is to be noted that in the present specification, “%”indicates a percentage by mass unless otherwise specified.

As described above, the fluororesin of the present invention is afluororesin comprising a substituted main frame resin having a structurein which some of hydrogen atoms and/or fluorine atoms bonded to carbonatoms of a main frame resin are substituted with an oxygen-containingfunctional group, and in which a visible light transmittance in awavelength of 600 nm of the fluororesin is greater than that of the mainframe resin.

Here, examples of the main frame resin include various fluororesins, butthe fluororesin of the present invention is used for a sliding purpose.Therefore, a fluororesin superior in sliding characteristic ispreferably used as the main frame resin among these fluororesins.Specifically, one or more of a tetrafluoroethylene resin, aperfluoroethylene polypropylene resin, a perfluoroalkoxy resin, and avinylidene fluoride resin are preferable.

As the oxygen-containing functional group substituted for at least oneof the hydrogen atoms and/or at least one of the fluorine atoms bondedto the carbon atoms of the main frame resin, any functional groupcontaining an oxygen atom may be used, and preferred examples include ahydroxyl group, a carbonyl group, a carboxyl group, a ketone group, anether group and the like.

One or more hydrogen atoms and/or one or more fluorine atoms may besubstituted with the oxygen-containing functional group. All of thecorresponding hydrogen atoms may be substituted, but at least onefluorine atom needs to remain, i.e., not all of the fluorine atomsbonded to the carbon atoms of the main frame resin are substituted.

It is to be noted that for the sake of convenience of description,“substitution” has been described, but it does not have any limitingmeaning attributable to a substitution reaction, e.g., the expressionthat an atom is “substituted” with a functional group indicates that thesubstituted main frame resin differs from the main frame resin in thatthe functional group is present instead of the atom in at least onelocation in the molecule (i.e., the expression “substituted” does notrequire that a particular reaction or any reaction have taken place inwhich the atom is replaced with the functional group at any location).Even when the atoms are substituted by an additional reaction, this isincluded in the scope of the present invention. Similarly, theexpression “extracted” as used herein, does not have any limitingmeaning, e.g., the expression that an atom is “extracted” from a mainframe resin indicates that the modified main frame resin differs fromthe main frame resin in that the atom is not present in the modifiedmain frame resin (i.e., the expression “extracted” does not require thata particular reaction or any reaction have taken place in which the atomis removed). In other words, the substituted main frame resins and themodified main frame resins of the present invention are definedchemically, and are not in any way limited in regard to how they wereproduced.

By the introduction of the oxygen-containing functional group, thewettability of the lubricating oil or the operating oil on the obtainedfluororesin is improved.

For example, when the oxygen-containing functional group is introduced,it is possible to obtain a surface energy of the obtained fluororesinwithin the range of from about 20 to about 40 dyne/cm, preferably about25 to about 35 dyne/cm.

In general, the surface energy of the lubricating oil or the operatingoil is in the range of from 25 to 35 dyne/cm. Therefore, when thesurface energy of the fluororesin is within the range of from about 20to about 40 dyne/cm, preferably from about 25 to about 35 dyne/cm, thewettability of the fluororesin with respect to the lubricating oiland/or the operating oil can be largely enhanced, and prohibition ofproduction of an oil film because of the wettability can be avoided inthe presence of the lubricating oil and/or the operating oil.

Moreover, in the fluororesin of the present invention, the visible lighttransmittance in the wavelength of 600 nm is greater than that of themain frame resin.

This increase of the visible light transmittance is a phenomenon causedby reduction of a crystal size of the fluororesin after irradiation withthe electrolytic dissociative radiation.

When the sliding member using the fluororesin of the present inventionslides, the crystal size is reduced. Therefore, a size of wear powder issmaller than with a conventional fluororesin. Moreover, fluidity ofcrystals increases, the crystals easily move/stick to a target material,production of a moved/stuck film is promoted, sliding between thefluororesins is caused, and therefore wear on the fluororesin itselfdecreases.

It is to be noted that the fluororesin of the present invention isusable as a solid lubricating material. In this case, the fluororesin ispreferably added to another resin at a ratio in the range of from about5 to about 70%.

When an addition amount is less than 5%, friction characteristic isreduced. On the other hand, even when the addition amount increases from70%, a friction characteristic improving effect is saturated, and theeffect is not improved any more.

Next, a method of reforming the fluororesin of the present inventionwill be described.

As described above, the method of reforming the fluororesin of thepresent invention is a method of manufacturing the fluororesin,comprising: extracting some hydrogen atoms and/or fluorine atoms bondedto carbon atoms of a main frame resin to introduce an unsaturated bond;and applying dissociation energy of the unsaturated bond in the presenceof oxygen.

Here, the step of introducing the unsaturated bond is not especiallylimited, but irradiation with an electrolytic dissociative radiation ina range of from about 1 to about 10 kGy is preferable in a state inwhich the bond is heated in an inactive gas atmosphere having an oxygenconcentration of about 10 Torr or less at not less than a melting pointof the main frame resin, while a component (e.g., another polymer) otherthan a component containing the same monomer as that of the main frameresin is not allowed to be present.

In this case, when the oxygen concentration exceeds 10 Torr, or aheating temperature is less than the melting point of the main frameresin, the unsaturated bond is not formed, main chain cutting of themain frame resin selectively occurs, and the main frame resin itself issometimes decomposed.

Moreover, when the intensity of the electrolytic dissociative radiationis less than 1 kGy, formation of the unsaturated bond is inhibited. Onthe other hand, when the intensity of the electrolytic dissociativeradiation exceeds 10 kGy, decomposition of the main frame resin itselfis sometimes promoted.

It is to be noted that the electrolytic dissociative radiation is notespecially limited, but an α-ray, a proton ray, a heavy ion, a β-ray, anX-ray, a γ-ray, a neutron ray and the like are usable.

On the other hand, in the step of dissociating the unsaturated bond, thedissociation energy of the unsaturated bond is preferably applied asheat energy preferably under an oxygen partial pressure of about 13.3kPa or more, but this is not especially limited.

In this step, the unsaturated bond introduced into the main frame resinis dissociated to react with the oxygen atoms, and changes into anoxygen-containing functional group such as a hydroxyl group, a carbonylgroup, a carboxyl group, a ketone group, or an ether group, and thefluororesin of the present invention is produced.

When the oxygen partial pressure is less than 13.3 kPa, a chemicalreaction between the unsaturated bond and the oxygen atom is inhibited,and a sufficient amount of the oxygen-containing functional group mightnot be formed in some cases. As a result, the wettability of thelubricating oil and/or the operating oil on the fluororesin is notimproved very much in some such cases.

It is to be noted that in the present invention, the heat energy appliedin the step of dissociating the unsaturated bond may be supplied duringmixing with the main frame resin or resin forming.

That is, as described above, a precursor of the fluororesin of thepresent invention obtained by introducing the unsaturated bond into themain frame resin may be used as a raw material of the resin forming, andthe heat energy may be applied to the material. Moreover, the materialmay be mixed with or formed into the main frame resin. Accordingly, thefluororesin of the present invention is manufactured, and simultaneouslymolded, and a resin molded article formed of the fluororesin of thepresent invention can be obtained.

A compound obtained by introducing the unsaturated bond into the mainframe resin in this manner is a precursor of the fluororesin of thepresent invention. In a molding step involving heating, the precursorfunctions as a molding material as such.

It is to be noted that the molding method in which the fluororesinprecursor of the present invention is usable simply as the moldingmaterial is not especially limited as long as the heating is performed.The examples include injection molding, ram molding, compression moldingand the like.

According to the present invention, a predetermined oxygen-containingfunctional group is introduced by an appropriate unsaturated bondintroducing process or the like. Therefore, there can be provided afluororesin which has both a low friction coefficient and a superiorwear resistance and which exhibits a superior friction characteristic inthe presence of a lubricating oil and/or an operating oil, a method ofreforming a fluororesin, a sliding member and a sliding devicecomprising such a fluororesin.

That is, by use of a fluororesin in a sliding member, a satisfactoryfriction characteristic can be realized in the presence of a lubricatingoil and/or an operating oil. The fluororesin is constituted bysubstituting some of the hydrogen atoms and/or fluorine atoms bonded tocarbon atoms of a main frame resin with an oxygen-containing functionalgroup, so that a visible light transmittance in a wavelength of 600 nmis increased as compared with that of a main frame resin.

Furthermore, according to the present invention, substantially by onlyan irradiation step of an electrolytic dissociative radiation, it ispossible to manufacture a fluororesin which exerts a satisfactoryfriction characteristic in the presence of a lubricating oil and/or anoperating oil, and therefore there is an advantage that investment intoimprovement of a sliding characteristic of the fluororesin may be small.

In the present invention, the introduction of the unsaturated bond andthe dissociation of the unsaturated bond can be confirmed from analysisresults of a Fourier transform infrared spectrophotometer (FT-IR), shownin Table 1 below.

That is, when irradiated with the electrolytic dissociative radiation, apeak area of a peak assigned to the unsaturated bond increases.Furthermore, when the step of dissociating the unsaturated bond isperformed, the increased peak area decreases. From this, it is confirmedthat the unsaturated bond has been produced and dissociated. TABLE 1Absorption peak area of double bond Article electrolytic irradiated withdissociative electrolytic radiation Wave number [cm⁻¹] Initialdissociative irradiation + (attributed peak) article radiationdissociation step 1785 (—CF═CF2) 1.08 1.24 1.12 1717 (—CF═CF—) 0.0520.065 0.054

When some of the hydrogen atoms or fluorine atoms bonded to the carbonatoms are substituted with the oxygen-containing functional groupthrough the above-described steps, the surface energy of the obtainedfluororesin can be set to within the range of from about 20 to about 40dyne/cm, preferably from about 25 to about 35 dyne/cm.

In general, since the surface energy of the lubricating oil or operatingoil is within the range of from about 25 to about 35 dyne/cm, thesurface energy of the fluororesin is set to be within the range of fromabout 20 to about 40 dyne/cm, preferably from about 25 to about 35dyne/cm. Consequently, the wettability of the fluororesin with respectto the lubricating oil and/or operating oil is largely enhanced, and theinhibition of the oil film production because of the wettability can beavoided even in the presence of the lubricating oil and/or the operatingoil.

On the other hand, as described above, the main frame resin isirradiated with the electrolytic dissociative radiation in a range offrom about 1 to about 10 kGy when heated at not less than the meltingpoint in the inactive gas atmosphere having an oxygen concentration ofabout 10 Torr or less. Then, simultaneously with the production of theunsaturated bond, the crystal size is reduced.

Specifically, as shown in FIG. 1, since the visible light transmittanceincreases after the irradiation of the electrolytic dissociativeradiation, it can be confirmed that the crystal size is reduced.

When the crystal size is reduced, the size of wear powder decreases in acase where the fluororesin is slid. Moreover, crystal fluidityincreases. Accordingly, the moving/sticking to a target material isfacilitated, formation of a moved/stuck film is promoted, thefluororesins slide on each other, and therefore wear on the fluororesinitself decreases.

Additionally, by the irradiation with the electrolytic dissociativeradiation, a bridging structure is formed in a molecule chain of thefluororesin. Even in this case, the wear resistance of the fluororesinis enhanced.

To sufficiently enhance the wear resistance by the above-describedreduction of the crystal size, the visible light transmittance in awavelength of 600 nm is set to preferably about 10% or more. When thevisible light transmittance in the wavelength of 600 nm is less than10%, the crystal size is not sufficiently reduced, and sufficient wearresistance is not obtained in some cases.

Next, a sliding member, a sliding apparatus and a sliding system of thepresent invention will be described.

In the sliding member of the present invention, the fluororesin of thepresent invention is applied to one or more sliding portions, and thesliding portion or portions preferably contain the fluororesin of thepresent invention at a ratio of from about 5 to about 70%.

When a content of the fluororesin is less than about 5%, a frictionimproving effect by the addition of the fluororesin is not sufficientlyexerted. When the content exceeds about 70%, the forming sometimesbecomes difficult.

Moreover, since the wettability of the fluororesin of the presentinvention with respect to the lubricating oil and/or operating oil isimproved as described above, the sliding member of the present inventionis also suitable for use in the presence of the lubricating oil and/oroperating oil. However, in this case, surface roughness of a slidingsurface of a sliding target material is set to Rz=about 10 μm or less,preferably Rz=about 5 μm or less.

When the surface roughness Rz of the sliding target material exceedsabout 10 μm, the fluororesin of the present invention is inhibited frombeing moved/stuck to the target material sliding face, a frictioncoefficient reducing effect is not developed, and conversely wear ispromoted.

It is to be noted that the sliding apparatus of the present inventioncomprises a sliding member of the present invention, and at least oneoil material selected from lubricating oils and operating oils. Asliding system of the present invention comprises a sliding apparatus ofthe present invention and a target material which comprises a slidingsurface on which the sliding member of the sliding apparatus slides.

A suitable surface roughness of the sliding target member is similar tothat of the sliding member.

EXAMPLES

The present invention will be described hereinafter in more detail inaccordance with examples and comparative examples, but the presentinvention is not limited to these examples.

Example 1

Tetrafluoroethylene superior in low friction among fluororesins was usedas the main frame resin, the molding powder (manufactured by Asahi GlassCo., Ltd., trade name G163) was irradiated with electron rays(pressurizing voltage of 2 MeV) in an atmosphere having an oxygenconcentration of 1 Torr, nitrogen concentration of 800 Torr on heatingconditions at 350° C. to introduce an unsaturated bond into the powder,and a targeted precursor of the fluororesin was obtained. Next, theprecursor was crushed with a jet mill until an average particle diameterreached about 20 μm.

A fluororesin precursor obtained by subjecting 90% of a resin (G163)(which was the same as the main frame resin used in the above-describedunsaturated bond introducing process) was added by 10%. A powder mixturewas sufficiently mixed with a mixer. Thereafter, the mixture was treatedat 300° C. for 12 hours, a high-temperature volatile component wasremoved, and a fluororesin mixture, that is, a mixture of one example ofthe fluororesin of the present invention and the main frame resin wasobtained.

Next, the obtained fluororesin mixture was preliminarily molded into acylindrical shape at a molding pressure of 50 MPa, and next fired in anelectric furnace at 350 to 400° C. for three hours. The obtainedfluororesin molded member was worked with a lathe, and a ring test piece10 having a straight abutment joint 20 shown in FIG. 2 was prepared.

Example 2

To 70% of the above-described main frame resin (G163), 30% of aprecursor fluororesin into which an unsaturated bond was introduced in amethod similar to that of Example 1 was added, and worked into a ringtest piece in the same manner as in Example 1.

Example 3

To 60% of the above-described main frame resin, 30% of a precursorfluororesin into which an unsaturated bond was introduced in the samemanner as in Example 1, and 10% polyamide imide powder (manufactured byAmoco Co., trade name Tohron 4203L, average particle diameter of 15 μm)were added. Thereafter, the material was worked into a ring test piecein the same manner as in Example 1.

Comparative Example 1

A fluororesin (G163) was preliminarily molded into a cylindrical shapeat 50 MPa, and thereafter fired in an electric furnace at 350 to 400° C.for three hours. The obtained fluororesin molded member was worked intoa ring test piece in the same manner as in Example 1.

Comparative Example 2

A fluororesin was subjected to a corona treatment in the atmosphere on acondition of 200 W·min/m², and a functional group was applied to thefluororesin. After performing the corona treatment, the fluororesin waspreliminarily molded into a cylindrical shape at a molding pressure of50 MPa, and thereafter fired in an electric furnace at 350 to 400° C.for three hours. The obtained fluororesin molded member was worked intoa ring test piece in the same manner as in Example 1.

Comparative Example 3

A fluororesin was subjected to a corona treatment in the atmosphere on acondition of 200 W·min/m², and a functional group was applied to thefluororesin. To 70% of the fluororesin subjected to the coronatreatment, 20% of graphite powder (manufactured by SEC Co., trade nameSGL 3 μm), and 10% of carbon fibers (manufactured by Kureha ChemicalIndustry Co., Ltd., trade name Kureha Chop M-2007S, fiber diameter of14.5 μm, fiber length of 90 μm) were added. The mixture was sufficientlymixed with a mixer.

Next, the mixture was preliminarily molded into a cylindrical shape at amolding pressure of 50 MPa, and thereafter fired in an electric furnaceat 350 to 400° C. for three hours. The obtained fluororesin moldedmember was worked into a ring test piece in the same manner as inExample 1.

Performance Evaluation

Contact Angle Measurement and Surface Energy Calculation

With respect to test pieces of Examples 1 to 3 and Comparative Examples1 to 3, a contact angle of pure water and methylene iodide was measured,and surface energy was calculated from a measured value. Obtainedresults are shown in Table 2. TABLE 2 Exam- Exam- Exam- Comp. Comp.Comp. Sample ple 1 ple 2 ple 3 Ex. 1 Ex. 2 Ex. 3 Surface 23 27 33 18 2425 energy (dyne/cm)

As shown in Table 2, in test pieces of Examples 1 to 3, surface energyis remarkably greater than in Comparative Examples 1 to 3. It has beenseen that some of the hydrogen atoms or fluorine atoms bonded to carbonatoms of the main frame resin are substituted with an oxygen-containingfunctional group by a combination of the unsaturated bond introducingprocess and the dissociation process of the unsaturated bond accordingto the present invention.

Sliding Characteristic Improving Effect

With respect to the test pieces of Examples 1 to 3 and ComparativeExamples 1 to 3, a friction test was conducted in the followingprocedure using an automatic shift operating oil (manufactured byIdemitsu Kosan Co., Ltd., trade name Matic J).

An aluminum die cast material (ADC-12) was selected as a target materialto be brought into sliding contact. To attach the material to a verticaltype pin-on-disk type frictional wear tester shown in FIG. 3, thisaluminum die cast material was worked into a disc 25, a shape had adiameter of 60 mm, and a thickness of 10 mm, and a surface roughness ofa sliding contact face was set to Ra=about 10 μm.

Here, a frictional tester shown in FIG. 3 will be described. This testerhas a ring holder 21 in an upper part thereof. This ring holder 21 isfixed by pressing a ring test piece outer peripheral face 17 b onto aholder groove portion by a spring force of a snap ring 22 disposed onthe side of a ring test piece inner peripheral face 17 a, and a ringtest piece 10 is prevented from being moved in a diametric directionduring a sliding time.

On the other hand, a disc holder 26 connected to a rotation shaft 27 isdisposed in a lower part of the tester. When a disc 25 is fixed to thedisc holder 26 via a bolt, the disc 25 is rotatable with respect to thering test piece 10. Next, when the ring holder 21 is lowered, the ringtest piece 10 is brought into sliding contact with the disc 25.Furthermore, when a pressure P is applied from an axial direction of thering holder 21, the ring test piece 10 is brought into pressing contactwith the disc 25. It is to be noted that in this case, a sliding contactportion between the ring test piece 10 and the disc 25 is immersed in anoperating oil (Matic J) 28 for the automatic shift.

Results of a friction test performed using the tester on test conditionsincluding pressing contact face pressure: 5 MPa, friction speed: 10m/second, test time: 6 hours are shown in FIG. 4 and Table 3.

It is seen from FIG. 4 that with respect to the test pieces of Examples1 to 3, a friction test can be continued for six hours, and a frictioncoefficient during the test is substantially stable. On the other hand,with respect to Comparative Examples 1 to 3, abnormal wear on the ringtest piece occurs during the testing.

Moreover, in comparison of Example 1 with Example 2, it is seen thatwhen an addition amount of the main frame resin subjected to thecombination of the unsaturated bond introducing process and theunsaturated bond dissociating process according to the present inventionis increased, the friction coefficient tends to drop. It is to be notedthat, as shown in Table 2, when the wettability with the lubricating oiland operating oil is improved by the increase of the surface energy, andan oil film forming capability is enhanced, a friction coefficientreducing effect is developed.

On the other hand, as to Example 3, PAI powder superior in wettabilitywith the lubricating oil or operating oil is added to the fluororesinwhose wettability has been improved as described above. Accordingly, theoil film formation is further promoted, and the friction coefficientdrops. TABLE 3 Measurement Comp. Comp. Comp. item Ex. 1 Ex. 2 Ex. 3 Ex.1 Ex. 2 Ex. 3 Wear volume 1.1 0.8 0.3 Abnormal Abnormal Abnormal (mm³)of ring wear wear wear test piece Wear depth 5.4 3.8 2.4 4.6 6.8 18.8(μm) of disc test piece

Table 3 shows wear amount measurement results of the ring test piece andthe disc test piece after ending the friction test. It is seen inExamples 1 to 3 that the wear amount of the ring test piece isremarkably reduced by the reduction of wear powder accompanying theconstruction of the bridging structure and the reduction of the crystalsize.

On the other hand, as to Comparative Examples 1 to 3, since testconditions themselves exceed PV limits of a general fluororesin,abnormal wear is generated in a comparatively short time (typicallyabout one hour) after test start.

FIG. 5 schematically depicts an embodiment of a sliding system 50 inaccordance with the present invention, including a target material 51and a sliding apparatus 52 comprising a sliding member 53 having an oilmaterial 54 coated on a sliding portion 55 of the sliding member 53. Thesliding member 53 slides (horizontally in FIG. 5) on a sliding surface56 of the target material 51 after the target material 51 has beenlowered to a position where the sliding surface 56 of the targetmaterial 51 contacts the sliding member 53.

FIG. 6 schematically depicts a further embodiment of a sliding systemaccording to the present invention namely, a sliding system 60 includinga target material 61 and a sliding member 62.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawings, itwill be understood by one skilled in the art that various changes indetail may be effected therein without departing from the spirit andscope of the invention as defined by the claims.

1. A fluororesin comprising a substituted main frame resin, saidsubstituted main frame resin comprising carbon atoms, at least onefluorine atom and at least one oxygen-containing functional group, inwhich at least one atom selected from the group consisting of hydrogenatoms and fluorine atoms bonded to carbon atoms of a main frame resin issubstituted with said oxygen-containing functional group, said at leastone fluorine atom being bonded to at least one of said carbon atoms ofsaid substituted main frame resin, said fluororesin having a visiblelight transmittance in a wavelength of 600 nm which is greater than avisible light transmittance in a wavelength of 600 nm of said main frameresin.
 2. The fluororesin according to claim 1, wherein said visiblelight transmittance in the wavelength of 600 nm of said fluororesin isabout 10% or more.
 3. The fluororesin according to claim 1, wherein asurface energy of said fluororesin is in the range of from about 20 toabout 40 dyne/cm.
 4. The fluororesin according to claim 1, wherein saidmain frame resin comprises at least one resin selected from the groupconsisting of tetrafluoroethylene resin, perfluoroethylene polypropyleneresin, perfluoroalkoxy resin and vinylidene fluoride resin.
 5. Afluororesin precursor comprising a modified main frame resin in which atleast one atom selected from the group consisting of hydrogen atoms andfluorine atoms bonded to carbon atoms of a main frame resin is extractedto introduce an unsaturated bond.
 6. The fluororesin precursor accordingto claim 5, wherein said main frame resin comprises at least one resinselected from the group consisting of tetrafluoroethylene resin,perfluoroethylene polypropylene resin, perfluoroalkoxy resin andvinylidene fluoride resin.
 7. A method of reforming a fluororesin, saidmethod comprising: extracting at least one atom selected from the groupconsisting of hydrogen atoms and fluorine atoms bonded to carbon atomsof a main frame resin to introduce an unsaturated bond; and applyingdissociation energy of said unsaturated bond in the presence of oxygen.8. The method of reforming a fluororesin according to claim 7, whereinin said extracting at least one atom to introduce an unsaturated bond, acomponent other than a component containing the same monomer as that ofthe main frame resin is not present.
 9. The method of reforming afluororesin according to claim 7, further comprising applyingdissociation energy of said unsaturated bond at an oxygen partialpressure of at least about 13.3 kPa.
 10. The method of reforming afluororesin according to claim 7, further comprising applyingdissociation energy of said unsaturated bond by heat energy.
 11. Themethod of reforming a fluororesin according to claim 10, furthercomprising at least one of mixing said fluororesin and molding saidfluororesin, wherein said heat energy is applied in at least one of saidmixing said fluororesin and said molding said fluororesin.
 12. Themethod of reforming a fluororesin according to claim 11, wherein saidmolding is selected from the group consisting of injection molding, rammolding and compression molding.
 13. A sliding member comprising asliding portion, said sliding portion comprising a fluororesin accordingto claim
 1. 14. The sliding member according to claim 13, wherein acontent of said fluororesin in said sliding portion is in a range offrom about 5 to about 70%.
 15. A sliding apparatus comprising a slidingmember as recited in claim 13 and at least one oil material, said oilmaterial being selected from the group consisting of lubricating oil andoperating oil, at least a portion of said oil material being in contactwith at least a portion of said sliding member.
 16. A sliding systemcomprising a sliding apparatus as recited in claim 15 and a targetmaterial, wherein the target material comprises a sliding surface whichhas a surface roughness of Rz=about 10 μm or less and wherein thesliding member slides on the sliding surface.